The present invention relates to acicular ferromagnetic alloy particles of improved particle properties, especially large specific surface and improved magnetic properties, especially high coercive force and large saturation magnetization which are suitable as a magnetic material for use in magnetic recording media. And, the present invention relates to a process for producing such acicular ferromagnetic alloy particles.
Recently, the improvements such as the miniaturizing and weight-lightening, the long-time recording and the like of the reproducing apparatus for magnetic recording, for example, video tape recorder, has been dramatically progressed. Accompanied by such improvements, the necessity for improving the qualities of the magnetic recording media such as magnetic tape, magnetic disk and the like has been more and more increasing. Especially, the magnetic recording media of high signal to noise ratio, little chroma noise and improved frequency characteristic have been requested. To fulfill such a request, the improvements of the magnetic material has been also requested since the above-mentioned specific properties of magnetic recording media depend upon the specific properties of the magnetic material, as described hereinafter.
To increase the signal to noise ratio of the recording media, the noise level due to the recording medium should be lowered. For that purpose, the micronization of the particle size of the magnetic particles and the improvements of their dispersibility in the vehicle and their orientation and packing in a coating medium are essential. Since the improvement of the latter brings also the surface smoothness to the recording media, such an improvement is important.
To reduce the chroma noise, the improvement of the surface smoothness of the recording medium is important, which depends upon the dispersibility in the vehicle and the orientation and packing in a coating medium of the magnetic particles.
To improve the frequency characteristic, the improvements of coercive force and residual magnetic flux density of the recording medium are important, each of which depends upon the coercive force and the saturation magnetization of the magnetic particles. Further, the latter depends upon their dispersibility in a vehicle and their orientation and packing in a coating medium.
As clear from the above-mentioned, it is necessary that the magnetic particles have excellent acicularity and the particle size thereof is uniform without the contamination of the dentrites and the apparent density thereof is as possible as large to improve their dispersibility in the vehicle and their orientation and packing in a coating medium. In addition, the specific surface area of the magnetic particle which is an index of the particle size and the coercive force as well as the saturation magnetization thereof are necessarily as large as possible.
As the magnetic material ferromagnetic iron oxide particles are conventionally employed in magnetic media, but the ferromagnetic iron oxide particles are not satisfactory in both their particle properties and their magnetic properties. The improved magnetic materials are acicular ferromagnetic iron particles and acicular ferromagnetic alloy particles obtained by heating acicular particles of .alpha.-iron(III) oxide hydroxide, acicular .alpha.-ferric oxide particles, each of these containing metal(s) different from iron in a flow of reducing gas at about 350.degree. C. Although the thus-obtained particles have relatively satisfactory magnetic properties, they have not excellent acicularity and uniformity in particle size and the dentrites are coexistent therein. This is due to the .alpha.-iron(III) oxide hydroxide as a starting material.
The most representative process for producing the acicular particles of .alpha.-iron(III) oxide hydroxide comprises blowing an oxygen-containing gas into an aqueous suspension containing precipitated ferrous hydroxide obtained by adding an aqueous alkali solution in an amount more than the equivalent into an aqueous solution of a ferrous salt at a temperature of lower than 80.degree. C. and at a pH of over 11 to effect the oxidation. However, the thus-obtained acicular particles of .alpha.-iron(III) oxide hydroxide are around 0.5 to 1.5 .mu.m in length of long axis and their axial ratio is at most 10:1 and the dentrites are coexistent therein. One of this cause is in that the particle sizes of both the flock of ferrous hydroxide which is the precursor of the acicular particles of .alpha.-iron(III) oxide hydroxide and the ferrous hydroxide particles themselves which constitute the flock of ferrous hydroxide are not uniform. Further, the other cause, is in that the stage of the formation of the nuclei of the acicular particles of .alpha.-iron(III) oxide hydroxide and the stage of the growth of the nuclei of acicular particles proceed simultaneously since the contact of ferrous hydroxide with dissolved oxygen to form the nuclei of the acicular particles of .alpha.-iron(III) oxide hydroxide is partial and uneven.
A method for preparing the acicular particles of .alpha.-iron(III) oxide hydroxide which have the uniformity in particle size and do not contain the dentrites is to add a water-soluble silicate such as sodium silicate, potassium silicate and the like in an amount of 0.1 to 1.7 atomic % calculated as the ratio of Si to Fe(II) and based on the amount of ferrous hydroxide in the suspension (hereinafter referred to as simply "calculated as the ratio of Si/Fe(II)") into the aqueous alkali solution or the aqueous suspension containing ferrous hydroxide in advance of the blowing of an oxygen-containing gas into the aqueous suspension containing ferrous hydroxide (refer to Japanese Patent Publication Nos. 8461/80 and 32652/80). By such an addition of the water-soluble silicate, the minute flocks of ferrous hydroxide and the minute particles of ferrous hydroxide can be obtained with uniform particle size, and further the nuclei of the acicular particles of .alpha.-iron(III) oxide hydroxide grow after the formation of the nuclei thereof is completed. Accordingly, the thus-obtained acicular particles of .alpha.-iron(III) oxide hydroxide have excellent acicularity and the particle size thereof is uniform without the contamination of the dentrites, they being to be said a favorable starting material. As a result, the acicular ferromagnetic alloy particles obtained by reducing the favorable starting material have also the uniformity in particle size without the contamination of the dentrites and so the apparent density thereof is satisfactory. The other properties such as the dispersibility, the orientation and the like except the specific surface area are also satisfactory. As the specific surface area of these magnetic particles is around 20 m.sup.2 /g, however, they are undesirable on the whole.
Accordingly, the object of the present invention is to provide the acicular ferromagnetic alloy particles having the above-mentioned specific properties necessary for use as the magnetic recording medium.
Another object of the present invention is to provide a process for producing such acicular ferromagnetic alloy particles.
These and other objects of the present invention will become more apparent from the following description.
The foregoing and the other objects of the present invention will be accomplished by the use of the .alpha.-iron(III) oxide hydroxide doped with silicon, chromium and nickel as the starting material.